The ever-increasing demand for wireless communication services, such as cellular mobile telephone (CMT), digital cellular network (DCN), personal communication services (PCS) and the like, requires the operators of such systems to attempt to make maximum effective use of the available radio frequency bandwidth. Consider, for example, that a system operator is typically allocated a geographic territory and a certain amount of bandwidth that affords the ability to transmit and receive on a particular number of different channel frequencies. In an effort to make the best use of the allocated frequency space, the geographic territory is divided into a number of sub-areas called cells. A number of radio base stations are deployed throughout the assigned territory, with there typically being one base station located in each cell. Transmission power levels are then kept low enough so that mobile units in adjacent cells do not interfere with each other.
The system operator then determines how to split up his allocated frequency channels among the cells. Often, an extensive study is necessary to determine how to best allocate the channels to the available cells. This study attempts to minimize the interference between adjacent base stations while also determining how to best reuse the channels, that is, how to best allocate respective channels to more than one base station, in order to maximize channel availability in the service area.
In the process of performing such a study, cellular system designers generally rely upon an idealized model of cell placement, which is usually assumed to be a grid of hexagonally shaped cells overlying the service area. The object of the frequency planning process is to reuse each frequency as often as possible within the service area. Cells which reuse the same frequency set in this manner are referred to as homologous cells.
In general, reusing a frequency in every Mth cell thus means that 1/Mth of all frequencies are available in any given cell. It is therefore desirable to select an M-cell frequency assignment pattern, with M being as small as possible, in order to increase the capacity for handling mobile units in each cell.
In one common cellular frequency reuse scheme, directional antennas are deployed at every alterative corner of a cell to illuminate each cell. In this manner, channel assignments may be made in a repeating pattern of seven cells, until the entire area under control of the service provider is covered.
U.S. Pat. No. 4,128,740 issued to Graziano and assigned to Motorola, Inc. discloses another frequency allocation plan. Each cell site in that scenario is divided into six sectors. The frequencies available to the service provider are then divided among the six sectors such that adjacent channel interference is avoided for a given sub-array of four cells.
Another pattern which provides a reuse factor as low as two is disclosed in U.S. Pat. No. 5,073,971 also assigned to Motorola, Inc. This is accomplished through asymmetrical positioning of the repeating patterns so that they radiate towards one another in alternating rows.
Such schemes which reduce the reuse factor to a minimum value are at odds, however, with the fact that as M decreases, so does the distance between the homologous cells, so that the amount of interference between users of the same frequency in different cells increases. This interference between users of the same frequency in homologous cells is called co-channel interference. The ratio between the power of the desired carrier signal C and the co-channel interference I is often referred to as the carrier to interference ratio (C/I). Thus, as the reuse factor M decreases, this C/I ratio is normally expected to increase.
It is accepted wisdom that the conventional seven-cell reuse pattern provides adequate C/I ratios between homologous cells in systems making use of the Advanced Mobile Phone Service (AMPS) protocol, which is dictated by that standard to be 17 decibels (dB).
However, the above-mentioned four-and three-cell reuse patterns are typically understood to fall short of providing the necessary carrier to interference level for AMPS. Such three and four-cell reuse patterns are thus typically believed to only be practical in systems making use of other transmission techniques such as digital cellular systems that use code division multiple access (CDMA) or time division multiple access (TDMA) to achieve coding gain. The coding gain provided by such systems thus provides the ability to tolerate a lower C/I ratio without degradation of the service provided to homologous cells.
What is needed is a way to genuinely obtain a high degree of channel reuse by partitioning the use of frequencies among sectors, without also imposing a need for coding schemes and the like to minimize adjacent channel interference.